The present invention relates to a method for purifying exhaust gases from a waste incineration plant by utilizing a sorption method in a circulating fluidized bed as well as an exhaust gas purification system.
In the case of waste incineration, the exhaust gases being generated during that process are usually purified by separating the pollutants contained therein, such as HCl, HF, SO2, nitrogen oxides and dioxin as well as dusts, in an exhaust gas purification system.
A possible method for separating pollutants such as HCl, HF, SO2 and dioxins from the exhaust gases is the dry sorption of the pollutants by utilizing a sorbent.
For this, a sorbent is usually introduced into a fluidized bed reactor, where it is put in contact with the exhaust gas in a circulating fluidized bed. The pollutants thereby sorb to the sorbent. Downstream from the fluidized bed reactor is a solids separator. Solid matter carried along in the exhaust gas and consequently also sorbent loaded with pollutant is separated therein. The separated solid matter is either discharged or returned to the fluidized bed reactor.
A corresponding method is described in the document EP-A-1 537 905. There, a first addition of the additive serving as sorbent takes place to the fluidized bed or following the fluidized bed ahead of the separation and a second addition ahead of the fluidized bed to an exhaust gas duct leading to the fluidized bed.
A number of methods have been suggested for the regulation of the amount of sorbent to be supplied as described below.
JP-A-2000107562 describes a method for the regulation of the amount of absorbing powder to be added by determination of the backwash cycles of a bag filter.
EP-A-0 173 403 describes a method for the regulation of the amount of sorbent to be introduced into an exhaust gas stream, wherein the HCl value is used as reference variable, from which the setpoint value is calculated in conjunction with the measured amount of exhaust gas and a temperature dependent stoichiometry value. In this method, the also determined SO2 content can be included as additional correction. The document does not clearly indicate if the HCl concentration and the SO2 concentration is measured in the raw gas or in the pure gas.
A control method in which only the HCl concentration in the pure gas is measured (“feedback” control) has the disadvantage that a required sorbent supply does not take place until an increased pollutant content in the emitted clean gas has already been detected. In order to meet this disadvantage, methods were suggested, in which in addition to the above described measurement in the pure gas, also the HCl and SO2 concentrations in the raw gas are measured, by way of which the theoretically required amount of sorbent can be determined (combined “feedforward/feedback” control).
However, it has been shown that the effectively required amount of sorbent can only be poorly determined by way of the measured concentrations of HCl and SO2. This relates to the fact that the circulating solid matter, which together with the actual sorbent comprises further solid components such as fly ash or fuel particles, has a not negligible residual sorption capacity. It is, for example, possible that in the case of a relatively low content of pollutants prior to the sorption, these are not completely separated despite a large amount of fresh sorbent being supplied, since the residual sorption capacity of the circulating solid matter is low and the amount of sorbent required for the treatment of pollutant peaks cannot be supplied to the system quickly enough. On the other hand, it is possible that in the case of a high residual sorption capacity of the circulating solid matter, the supply of fresh sorbent is not even required at all when the pollutant content prior to the sorption is relatively high. As a result of the composition, which varies greatly depending on the composition of the combusted waste, of the exhaust gas and of the entrained solids, the residual sorption capacity can not be determined numerically. If hydrated lime is used as sorbent, then the sorption capacity is additionally influenced by carbonation reactions, which makes a numerical determination of the residual sorption capacity completely impossible.
A further disadvantage of the existing methods for the purification of exhaust gases from waste combustion is found in their lack of operational reliability. The reason for this is that the chloride content of the circulating solid matter strongly fluctuates and, for a specific content in the solid matter, leads to sticking and caking, which, in the extreme case, can completely clog the exhaust gas purification system.